Despite having high strength, carbon-carbon composites (e.g., carbon fiber reinforced polymers (CFRPs)) exhibit low wear resistance, low thermal and electrical conductivity that limit their use in demanding applications. In addition, the use of carbon-carbon composites in high temperature applications has been limited by the inherent problem of susceptibility to oxidation, among other drawbacks. To address the current deficiencies that are mainly related to surface characteristics of CFRPs different surface coatings can be applied. However, one of the main challenges in coating (e.g. thermal spray coating technology) the CFRP surface is the degradation of epoxy used in CFRPs. Many composite materials which contain a mixture of organic and inorganic material can be damaged or otherwise altered during coating processes. This is particularly true for high-energy/high temperature coating processes which otherwise would provide enhanced properties to the composite material. Thus, many composite materials cannot be used in more demanding applications.
To address the oxidation, the concept of functionally graded materials (FGMs) has been demonstrated. Thus, any surface coatings used for protection of CFRPs require to demonstrate good adhesion and improved functionality. One technique to achieve good adhesion as well as improved functionality is to grade the chemistry of coating material (FGM) in a way that exhibits good adhesion in first layer in contact with CFRP and improved functionality (e.g. wear resistance) in outermost layers. However, FGMs are often utilized as thin film coatings instead of bulk-FGMs, due to difficulty in controlling the gradient, and have been demonstrated mainly for metal substrates. Other limitations include poor adhesion of thermal spray coatings on such composites. Coating of different materials on such composites typically requires plasma treatment and/or etching, chemical and/or mechanical treatment of the surface, each of which has drawbacks and/or adds cost and processing time to finished articles.